1. Field of the Invention
The present invention relates to a motor-driven air pump system for supplying secondary air to an engine exhaust air passage to purify the exhaust gas of a vehicle engine, and the present invention particularly relates to an improvement in a check valve used in such a system.
2. Description of the Related Art
A motor driven air pump system for forcibly supplying secondary air into an engine exhaust air passage is known as shown in Japanese Patent Application Laid Open No. Hei 4-219415. Such a conventional motor-driven air pump system is constructed as shown in FIG. 3. In this Figure, reference numeral 1 designates an engine, 2 a suction passage, 3 an air filter, 4 a throttle valve disposed in suction passage 2, 5 a suction manifold branching off from suction passage 2, 6 a cylinder, 7 a combustion chamber, 8 an exhaust manifold, 9 a catalytic converter, and 10 a combined exhaust gas passage. Reference numeral 11 designates an air pump which comprises a motor unit 11a operating when it receives a control signal from an electronic control device (not shown) having a microprocessor and a pump unit 11b which compresses air sucked through an intake air passage 12 from a point downstream from air filter 3. A secondary air passage 14 is connected to the discharging side of pump unit 11b through a passage switching valve 13 having a structure to be explained later, secondary air passage 14 diverges to become secondary air branch passages 15, and the end of secondary branch passages 15 open to exhaust gas manifold 8. In each secondary air branch passage 15, a check valve 16, the structure of which is shown in FIG. 4 on a enlarged scale, is disposed to pass only the secondary air passing therethrough toward exhaust manifold 8 (from the left to the right in FIG. 4). In other words, in FIG. 4, which shows a conventional check valve 16 used in a conventional motordriven air pump system, an approximately cylindrical valve housing 17 has a funnel-shaped inlet cylindrical portion 19 tightly secured thereto together with a disk partition plate 18, which is formed with a plurality of sector valve openings 20. A rubber valve disk 21 is secured to the center of partition plate 18 by a rivet 24 through a partspherical stopper 22 and a backup coil spring 23. Reference numeral 25 designates a bolt portion formed integral with the outlet opening of valve housing 17. The structure of a conventional passage switching valve 13 is shown in FIG. 5. On a valve housing 26, an inlet cylindrical portion 27 connected to the secondary air passage 14 and an outlet cylindrical portion 28 connected to the discharging side of the motor-driven air pump 11 are formed. A valve opening is formed at the end of outlet cylindrical portion 28. A valve hood 32 is secured together with a diaphragm 31 to the opening end of a valve chamber 30 formed in valve housing 26. As a result, a pressure control chamber 33 is formed at the upper portion of diaphragm 31 in valve hood 32. A valve disk 35 to which a rubber sealing member 34 is thermally bonded is secured at one side of diaphragm 31 to close valve opening 29, and a deep bottomed pressure plate 36 is secured at the other side of diaphragm 31 to a hole at their center by a rivet 37. Pressure plate 36 is provided with a rubber ring 40 since it may collide with the inner surface of valve hood 32 which functions as a stopper. A compression spring 38 seated on pressure plate 36 held in valve hood 32 biases diaphragm 31 and valve disk 35 to close valve opening 29. As shown in FIGS. 1 and 3, pressure control chamber 33 formed in valve hood 32 is connected to a vacuum switching valve (VSV) 42 through a control pressure inlet 39 and control pressure intake pipe 41. VSV 42 is of an electromagnetic type which has one port connected to the downstream portion (surge tank 44, for example) of throttle valve 4 through a vacuum intake pipe 43 and another port 45 open to the atmosphere. VSV 42 is controlled to switch on or off according to control signals generated by the aforementioned electronic control device (not shown) to supply either the vacuum or the air, as the control pressure, to control pressure chamber 33 of passage switching valve 13 through control pressure intake pipe 41.
Since the conventional motor-driven air pump is constructed as above, when engine 1 is made to start at a cold temperature, the electronic control device (not shown) drives VSV 42 to connect control pressure intake pipe 41 to vacuum intake pipe 43. Consequently, the pressure in control pressure chamber 33 of passage switching valve 13 decreases to a negative pressure as the vacuum at the downstream of throttle valve 4 in engine suction passage 2 is introduced, and diaphragm 31 is sucked to move against compression spring 38 so that valve disk 35 moves to open valve opening 29. At the same time the control device generates a control signal and energizes electric motor 11a of motor-driven air pump 11 to rotate pump unit 11b, which compresses the secondary air sucked therein from suction passage 12 to flow through the opened passage switching valve 13, secondary air passage 14 and secondary air branch passage 15, and to deform rubber valve disk 21 of check valve 16, thereby opening valve opening 20 and to permit the second air to flow into intake air manifold 8 of engine 1.
When engine 1 starts at a cold temperature and the secondary air is added to the exhaust gas flowing into catalytic converter 9, oxidization, or exothermic reaction, of HC and CO in the exhaust gas is expedited so that temperature of the catalyst of catalytic converter 9 rises sharply to activate the catalyst quickly, thereby to attaining complete purification of the exhaust gas. When warming-up of the catalytic converter 9 has been completed, the control device switches over VSV 42 to introduce atmospheric pressure through opening 45 into control pressure chamber 33 of passage switching valve 13 through control pressure intake pipe 41, so that valve disk 35 is biassed by compression spring 38 to close valve opening 29 and deenergize motor unit 11a of motor-driven air pump 11 to stop operation of pump unit 11b. As a result, the conventional motor driven air pump stops, and pump unit 11b becomes a simple secondary air passage to allow the secondary air therethrough.
Further, as the exhaust gas passes intermittently the exhaust gas passage right at the downstream of the exhaust valve such as at exhaust manifold 8, the exhaust gas pressure always pulsates, so that when engine 1 rotates slowly, there intermittently exists a momentary period in which the exhaust gas pressure becomes negative.
Another prior art system, in which a forcible secondary air supplying means such as a motor driven air pump is not used and a unpowered simple secondary air supplying system in which secondary air is introduced through a reed-type check valve is also well-known to the public.
In Japanese Patent Application Laid Open No. Hei 5-209512, there is disclosed still another secondary air supplying system for an engine which includes a control device for supplying secondary air of a quantity appropriate for a given to engine operating condition into an engine exhaust gas system. The system comprises a control unit in which a characteristic curve of a voltage applied to a motor is selected from a table previously stored in a memory to get the secondary air quantity appropriate for the engine operating condition where a control signal is generated based on the characteristic curve, and a driving unit for driving a DC motor of a motor-driven air pump according to the signal to make the current consumption of the DC motor correspond to the secondary air quantity designated by the signal. A reed valve is disposed at the downstream portion of the motor-driven air pump. In this prior art system, when the driving unit drives the DC motor of the motor driven air pump, the motor current is detected as a parameter and controlled responsive to the secondary air supplying quantity designated by the operation signal so that a desired control which is simpler and more precise than a control operation based on the motor rotational speed, particularly at the lower speed of the motor, may be provided.
The aforementioned conventional motor driven air pump system forcibly supplies the secondary air to the exhaust gas flowing through exhaust pipe 8 in order to expedite warming-up of catalytic converter 9 during cold starting of engine 1. However, that in the ordinary engine operation at such engine operating condition suitable to the object of exhaust gas purifying by introducing secondary air into the exhaust gas as the condition at the deceleration of engine 1 when throttle valve 4 is closed and HC and CO in the exhaust gas increases. In such a condition, passage switching valve 13, as in the warming-up of catalyzer 9, and the secondary air is supplied forcibly into exhaust manifold 8 by motor-driven air pump 11, thereby eliminating a bad smell of the catalyst generated by unburned gases such as HC and CO entering the catalytic converter 9 during deceleration. However, deceleration occurs frequently in the ordinary operation of engine 1 of a vehicle and if motor-driven air pump 11 is energized during each deceleration, the heat generation at motor unit 11 is so significant that the durability of motor driven air pump 11 is lowered. If motor driven air pump 11 is arranged to be durable and larger in size, the whole size of the system including motor-driven air pump 11 increases and the cost of the system is also unavoidably increased.
In such condition where secondary air supply is preferable, when motor-driven air pump 11 is made to stop and, at the same time, only passage switching valve 13 is made to open, check valve 16 opens during the negative pressure period caused by the exhaust gas pulsation. One may think that the secondary air may be introduced into the exhaust gas without power assistance under such circumstances.
However, check valve 16 of the conventional motor-driven air pump system employs rubber valve disk 21 to check the backflow of the exhaust gas and closes valve opening 20 by means of the rubber elasticity of valve disk 21 and the bias of coil spring 23, thus making the valve opening pressure high. Further, if the check valve 16 is opened during the momentary negative pressure period caused by the exhaust gas pulsation, the valve is not lifted enough the air flow resistance becomes high, and the pressure loss increases. Consequently, even when the momentary negative pressure caused by the pulsation of the exhaust gas pressure in exhaust manifold 8 is produced in valve housing 17 repeatedly, valve disk 21 may not follow the pulsation, and valve opening 20 cannot pass a sufficient amount of the secondary air therethrough.
In the conventional motor-driven air pump system, as shown in FIGS. 3-5, it is impossible to supply the secondary air without power assistance. If the secondary air supply was made by motor-driven air pump 11, it would be impossible to supply the secondary air during acceleration frequent supply is necessary, without a bigger motor-driven air pump 11. In the motor-driven air pump system shown in Japanese Patent Application Laid Open No. Hei 5-209512, since the motor driving electric current is used as the control parameter in order to control the discharge amount of the motor-driven air pump, it is necessary to control the motor according to an applied voltage characteristic curve.